Thermodynamic and transport properties of Ca-doped nickel oxide and relevance to the oxidation of CaO-coated-nickel

The electrical conductivity and thermoelectric power of CaO (2 and 2.6 mol%)-doped Ni_1-xO single crystals have been measured in the temperature range 1000-1400 C, as a function of oxygen partial pressure. These results show that Ca²⁺ leads to the shift of the Ni/Ni_1-xO phase boundary to higher PO₂, to a doping effect and to a mixed conductivity (e' and h), at PO₂ b 10^-4 atm. From electrical conductivity measurements in transient state, it was found that the chemical diffusion coefficient and the cationic vacancy diffusion coefficients are higher in the doped samples than in undoped Ni_1-xO. Furthermore, kinetic de-mixing experiments in an applied electric field show that D(Ca) ≥ D(Ni), due to the decisive role of correlation effects in the Ca-doped samples. These results have allowed us to explain the beneficial influence of CaO coatings on the oxidation of Ni polycrystals, at T ≤ 1200 C. It was found that the key features are the decrease of the driving force of diffusion due to the shift of the Ni/NiO phase boundary to higher PO₂, which increases with the amount of calcium, and blocking effects due to CaO precipitates near the position of the original metal surface, all the more important that the temperature is low and the oxidation time short. (authors)